doc: move env var section to development section

Content about environment variables belongs under introductionary
section.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/guides/env_vars.rst b/doc/develop/env_vars.rst
similarity index 100%
rename from doc/guides/env_vars.rst
rename to doc/develop/env_vars.rst
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 89908b0..82b3da1 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -8,6 +8,7 @@
 
    getting_started/index.rst
    beyond-GSG.rst
+   env_vars.rst
    application/index.rst
    modules.rst
    west/index
diff --git a/doc/guides/index.rst b/doc/guides/index.rst
index 3af90d8..81ca9fd 100644
--- a/doc/guides/index.rst
+++ b/doc/guides/index.rst
@@ -10,7 +10,6 @@
    coccinelle.rst
    code-relocation.rst
    flash_debug/index
-   env_vars.rst
    pinctrl/index
    emulator/index.rst
    platformio/index.rst